Sodium ion batteries are favourable candidates inter alia for future hybrid and electric vehicles because of the low material price of sodium and high energy density, beyond that available for lithium ion batteries. Sodium layered oxides (NaxMO2, M: Transition metal) can be used as materials for the positive electrode in view of their high capacity and ion diffusivity, leading to high energy and power batteries.
JP-2002-220231 discloses a layered sodium-cobalt-manganese oxide having a uniform composition and a single crystal for a lithium ion secondary battery. The oxide has the formula NaxCoyMn1-yO2 (0.6≤x≤0.8, 0.4≤y≤0.6). The materials prepared do not include Ni (nickel).
WO 2014/132174 discloses a doped sodium manganese cathode material for a sodium ion battery with the formula NaxMO2 wherein M=Mn1-y-zLiyAz, 0.66≤x≤0.95, y<0.33, z<0.2, A=Ti, Fe, Ni, Mg, Co. A number of Na—Li—Mn multi-cation compositions were investigated.
US 2012/0183837 discloses materials suitable for use as a cathode in a sodium electrochemical cell or battery, the electrode comprising a layered material of formula NacLidNieMnfMzOb, wherein M comprises one or more metal cations, 0.24≤c/b≤0.5, 0<d/b≤0.23, 0≤e/b≤0.45, 0≤f/b≤0.45, 0≤z/b≤0.45, the combined average oxidation state of the metal components (i.e., NacLidNieMnfMz) is in the range of about 3.9 to 5.2, and b is equal to (c+d+Ve+Xf+Yz)/2, wherein V is the average oxidation state of the Ni, X is the average oxidation state of the Mn, and Y is the average oxidation state of the M in the material.
JP-2012-250895 discloses a sodium-containing composite metal oxide having a layered crystal structure suitably used as an electrode active substance for a sodium secondary battery. The method for manufacturing an inorganic material includes converting at least a part of a lithium-containing composite metal oxide represented by LiaAbMcOd to a sodium-containing composite metal oxide having a layered crystal structure by an electrochemical treatment, wherein a is at least one element selected from the group consisting of Na and K; M is at least one transition metal element; 0<a≤1.5, 0≤b<1.5, 0<c≤3, 0<d≤6 and 0<a+b≤1.5.
There is an ongoing need in this field to provide composite metal oxides for sodium batteries which enable excellent and durable electrochemical properties, such as high cyclability.